The fact that water dissociates to form H⁺ and OH^- ions in a reversible reaction is the basis for an operational definition of acids and bases that is more powerful than the definitions proposed by Arrhenius. In an operational sense, an acid is any substance that increases the concentration of the H⁺ ion when it dissolves in water. A base is any substance that increases the concentration of the OH^- ion when it dissolves in water.
These definitions tie the theory of acids and bases to a simple laboratory test for acids and bases. To decide whether a compound is an acid or a base we dissolve it in water and test the solution to see whether the H⁺ or OH^- ion concentration has increased.

Information provided by: http://chemed.chem.purdue.edu